1. Field of the Invention
The present invention relates to a liquid crystal display device (hereinafter LCD), more particularly to an LCD having a wide viewing angle without color shift.
2. Background of the Invention
An LCD has the characteristics of light weight, thin thickness and low power consumption. It has been used in various information display terminals or visual equipment. The major operating mode for the LCD can be the twisted nematic("TN") and the super twisted nematic("STN"). Though they are presently commercially used in various LCD means, the characteristic of narrow viewing angle has still remained unsolved.
An In-Plane Switching(IPS) mode LCD has been suggested to solve the foregoing problems.
FIG. 1 is a plane view showing a general IPS mode LCD. A common electrode 11 is formed on the substrate 10 in the form of comb. A pixel electrode 13 is also formed on the substrate 10 in the form of comb, the comb-like electrodes 11 and 13 form a teeth shape. When an external voltage is applied between the common electrode 11 and the pixel electrode 13, an electric field F1 is formed therebetween and the liquid crystal molecules 15 are moved such that their long axes are parallel to the electric field F1.
There is a color shift in the direction of long and short axes due to the optical anisotropy since the long and short axes of liquid crystal molecule are different from each other in length.
As well known, the refractive anisotropy(or birefringence, .DELTA.n) has occurred due to the difference of the lengths of the long and the short axes. The refractive anisotropy(.DELTA.n) is also varied from the observer's viewing directions. Therefore a predetermined color is appeared on the region where the polar angle if 0 degree and the azimuth angles range of degrees 0, 90, 180 and 270 in spite of the white state. This results in color shift and more detailed description thereof is attached with reference to the equation 1. EQU T.apprxeq.T.sub.0 sin.sup.2 (2.chi.).multidot.sin.sup.2 (.pi..multidot..DELTA.nd/.lambda.) equation 1
wherein,
T : transmittance; PA1 T.sub.0 : transmittance to the reference light; PA1 .chi.: angle between an optical axis of liquid crystal molecule and a polarizing axis of the polarizing plate; PA1 .DELTA.n: birefringence; PA1 d: distance or gap between the upper and lower substrates(thickness of the liquid crystal layer); and PA1 .lambda.: wavelength of the incident light.
So as to obtain the maximum transmittance T, the .chi. should be .pi./4 or the .DELTA.nd/.lambda. should be .pi./2 according to the equation 1. As the .DELTA.nd varies with the birefringence difference of the LC molecules from viewing directions, the .lambda. value is varied in order to make .DELTA.nd/.lambda. to be .pi./2. According to this condition, the color corresponding to the varied wavelength .lambda. appears.
Accordingly, as the value of .DELTA.n relatively decreases at the viewing direction "a" toward the short axes of the LC molecules, the wavelength of the incident light for obtaining the maximum transmittance relatively decreases. Consequently a blue color having shorter wavelength than a white color is emerged.
On the other hand, as the value of An relatively increases at the viewing direction "b" toward the short axes of the LC molecules, the wavelength of incident light relatively increases. Consequently a yellow color having a longer wavelength than the white color is emerged. This causes deterioration to the resolution of IPS-LCDs.
To prevent the above disadvantages caused by color shift, a method for forming multi-domain within a unit pixel has been suggested. According to this method, one portion of an alignment layer is rubbed to a first direction, the other portion of the alignment layer is rubbed to second direction symmetrized the first direction. This method, however, is required additional masks compared to the conventional process of liquid crystal molecules alignment.